The invention relates to an antenna which is intended especially for radio telephones and the directional pattern of which can be altered controllably. The invention further relates to a radio telephone having such an antenna.
It is usually advantageous for the operation of a two-way radio apparatus if the transmitting and receiving characteristics of its antenna are good in all directions. In practice, antenna efficiency in transmitting and receiving varies depending on the direction, often drastically. In mobile communications networks, multipath propagation reduces the disadvantage caused by the unevenness of the antenna directivity pattern, but naturally it is preferred that the directivity pattern is as even as possible. As regards transmitting characteristics in communications devices held against the user's ear, it is considered undesirable that radiation is directed towards the user's head. Therefore, an ideal mobile telephone antenna receives well from all directions but transmits weakly into the sector where the user's head is located when the phone is placed in its normal operating position.
As regards receiving, the above-mentioned desirable characteristics are achieved with a whip antenna having a large ground plane, for its directivity pattern is circular on the plane perpendicular to the axis of the whip. In practice, the ground plane is the body of the radio apparatus, which is relatively small and indefinitely shaped from the antenna standpoint. Therefore, the directivity pattern may have considerable alternation. Likewise, as regards transmitting, the shape of the directivity pattern of a conventional whip antenna of a mobile phone varies uncontrollably so that radiation is directed towards the user's head, too.
Structures are known from the prior art where the antenna field is attenuated in the direction of the user's head by means of an additional element. FIGS. 1a,b show an example of such a structure. FIG. 1a shows a portion of the body 110 of a mobile phone and, above that, a small antenna circuit board 120 on the front side of which there is a meander-type radiating element 121. This is connected by its bottom end to the antenna port through a feed conductor 125. On the back side of the circuit board 120, shown in FIG. 1b, there is a conductive patch 122 which covers a major part of the radiating element. When the phone is in the use position, the conductive patch 122 stands between the radiating element and the user's head. The conductive patch is connected to signal ground GND so that it does not function as a significant parasitic radiator. Instead, it functions as a surface reflecting radio waves, attenuating radiation in the direction of the user's head. A drawback of this solution is that also the reception characteristic of the antenna deteriorates in said direction.